1. Field of the Invention
The present invention relates to a coaxial injector for a rocket engine and more particularly, to a tripropellant coaxial injector which allows for smooth transitioning between two types of propellants that are adapted to be alternatively mixed with a third propellant in a combustion chamber of a rocket engine.
2. Description of the Prior Art
Bipropellant rocket engines are known which combine a single oxidizer propellant with a single fuel propellant in a combustion chamber in a rocket engine to form a combustion reaction to produce thrust. Examples of such bipropellant rocket engines are disclosed in U.S. Pat. Nos. 3,699,772 and 4,206,594, assigned to the same assignee as the assignee of the present invention and hereby incorporated by reference. Such bipropellant rocket engines include a coaxial injector which includes a hollow pintle and a concentrically mounted annular outer sleeve. One end of the pintle is disposed within the combustion chamber. Intermediate the end of the pintle within the combustion chamber is a fuel orifice formed by the outer annular sleeve and the outer surface of the pintle. A first propellant, such as an oxidizer, is introduced within the pintle and is diverted in a generally radial direction by way of a cone-shaped projection and alternating slots formed on the end of the pintle. Fuel is introduced through the annular orifice defined by the outer annular sleeve and exterior surface of the pintle to enable the fuel propellant to be introduced into the combustion chamber in an axial direction. Such a configuration allows the fuel and oxidizer propellants to mix in the combustion chamber.
It is also recognized and understood in the art that it is sometimes practical and advantageous to have the oxidizer as the outer flowing propellant and the fuel as the propellant flowing internal to the pintle.
There is a desire to increase the performance of rocket launchers and spacecraft through the use of high density propellants to minimize vehicle tank volume and inert weight. For an atmospheric booster flight, minimizing volume also minimizes the drag loss inefficiency of the rocket. However, compared to low density high performance propellants, such as liquid hydrogen/liquid oxygen, high density propellants usually have lower combustion temperatures and higher molecular weight exhaust that leads to low rocket engine propellant use efficiency. Unfortunately, most high performance rocket engines commonly used in the art in such applications are bipropellant and thus only use two propellants. As such, it is relatively impossible to achieve the theoretical optimum performance for a given rocket vehicle size and mission because combustion performance must always be traded against propellant density. A single rocket engine that can operate with three or more propellants can permit an optimum selection of average performance versus average propellant density for a given vehicle size and mission.